Antialiasing in the context of digitizing line art and certain graphical image structures is best known as a method of using intermediate levels of intensity to achieve subpixel position of edges for several reasons including reduction or elimination of jaggies on the edges of lines and polygons, including text. Jaggies are primarily visible at the edges of sloped lines approaching horizontal or vertical. The term antialiasing suggests an analog term aliasing; normally representing the presence of low frequencies resulting from sampling high frequency signals at too low a sampling rate.
Consider a near-vertical (or near-horizontal) line segment. To be perfectly reproduced in a printed media, the phase, which represents the location of the edge, must continuously vary along the length of a segment. Due to the inherent sampling of a bi-level display or printed output, the phase exhibits jump discontinuities. Thus, this form of aliasing artifact leads to an induced jagged appearance where the structures are referred to jaggies. Within a sampled image any graphical object is eventually approximated as a polygon or collection of polygons. These polygons have straight edges, some of which will exhibit aliasing (jaggies and other placement defects). FIG. 1, for example, shows aliasing in two dimensions. When the triangle on the top of FIG. 1 is rasterized, the edges are aliased as reproduced in the triangle shown at the bottom of FIG. 1. In particular, the position along the bottom edge should move up slightly from column to column as one looks from left to right in the image at the bottom of FIG. 1. However, the position is quantized, as illustrated, producing the jagged appearance along the bottom of the triangle. Visibility of the anti-aliased image artifacts is increased by the regular nature of the jaggies, again a result of sampling.
Consider the following systems and their capability, or incapability, to utilize antiliased pixels. Xerox's Docucolor 40, for example, employs a high frequency analog line screen to render antialiased pixels, but that is not an option for some products or market segments. When conventional screens (e.g., approximately equal to 130-150 CPI dots) are employed in a rendering module, antialiased pixels are half toned and printed, resulting in objectionable halftone dots positioned along character edges. Hyperacuity printing techniques, for example those described by Curry, et al., (U.S. Pat. No. 5,138,339 and U.S. Pat. No. 5,485,289) can provide rendering for antialiased pixels that is compatible with simultaneously printing dot screen halftones in enhanced line art. However, these techniques require the use of tags to identify the antialiased pixels as antialiased line art. In the preferred embodiments described with respect to the present invention the rendering architecture distinguishes binary text/line from contone images to appropriately treat both image types. As will be described herein an algorithm or method may be employed in a rendering module or in other components of the rendering device to produce gray antialiased pixels of a form suitable for xerographic printing.
Antialiased images can be generated by capturing the image at a resolution greater than the final or desired output resolution, then reducing the resolution of the image by sub-sampling using an averaging process. A major benefit of antialiased images is that high contrast, saturated objects are surrounded with pixels possessing intermediate values that visually suggest the true, higher resolution position of object edges.
For example, in binary printing systems, such as many xerographic or ink jet systems that use a halftoning process to simulate continuous tone images, the antiliased edge pixels should be rendered with a very high frequency cell, ideally one having the resolution of the final output image. If the standard system halftone dot were to be used, the antialiased edges would be serrated or jagged at the standard halftone frequency. This rendering would reduce or even negate any value obtained through antialiasing. The use of a very high frequency screen over the entire antiliased image renders the antialiased pixel properly, but tends to sharpen the tonal curve and provoke print quality defects in the overall image. Also, distinct from the large area tonal reproduction curve (TRC), antialiasing may be employed so that the antiliased tonal reproduction curve meets any edge-pulling requirements specific to the output system (as opposed to a simple "averaging" process).
Hence, the present invention is directed to a method for tagging antialiased pixels that is applied within the architecture of a rendering device, and particularly, within a raster image processor (or digital front end) of such a system. Implementation of such a system in the early stages of processing, will enable the accurate and reliable identification of the antialiased pixels during later processing, as well as eliminate any need to subsequently attempt to characterize antialiased segments of the image with inherently less information about the image. Accurate identification of antialiased pixels enables the antialiased pixels to be rendered in a manner distinguishable from, and more appropriate than, the manner applied to continuous tone segments of an image.
Heretofore, a number of patents and publications have disclosed information related to antialiasing, the relevant portions of some may be briefly summarized as follows:
U.S. Pat. No. 5,646,751 to Motamed et al., issued Jul. 8, 1997, teaches a method for improving the speed of a color conversion operation using pixel tagging.
In "A Comparison of Antialiasing Techniques," IEEE CG&A, Vol. 1, No. 1, January 1981, pp. 40-48, F. Crow teaches that prefiltering is a computationally effective technique for antialiasing.
In accordance with the present invention, there is provided in a system for processing document images including both text/line art regions, and pictorial regions, a method for antialiasing segments of the image, including the steps of: receiving the document image; applying an anti-aliasing filter to text/line art regions within the image, to modify the image in a manner that suppresses aliasing effects; coincidentaly identifying those pixels modified by the antialiasing filter; and providing an indication for the identified pixels, said indication being detectable in a subsequent processing operation.
In accordance with another aspect of the present invention, there is provided, in a printing system, having a digital front end for processing document images including both text/line art regions, and pictorial regions, a method for antialiasing segments of the image, including the steps of: receiving the document image for printing on a medium; applying an antialiasing filter to text/line art regions within the image, to modify the image in a manner that suppresses aliasing effects present therein; coincidentaly identifying those pixels modified by the antialiasing filter; and providing an indication for the identified pixels, said indication being detectable in a subsequent processing operation.
One aspect of the invention deals with a basic problem in the rendering of antialiased images, particularly in text and line art regions--identifying the antialiased image segments on a pixel level. This aspect is further based on the recognition that the identification is preferably accomplished at the time of raster image processing; when the antialiasing occurs and may be indicated with a tag or similar indicia associated with a pixel or similar small image segment. The technique further employs a rendering architecture that, upon detecting indicia that a particular segment of an image has been antialiased, directs the rendering operation accordingly.